This invention relates to luminescent materials. More particularly, it is concerned with luminescent phosphors comprising a solid solution of zinc silicate and lithium zinc fluoride, a method of producing such phosphors, and cathodoluminescent screens employing such phosphors.
Manganese activated zinc silicate is a well known green-emitting cathodoluminescent phosphor identified commercially as type P-1. When manganese is incorporated into zinc silicate, the resulting phosphor is often off-white or dark in appearance. A white-bodied manganese-activated zinc silicate phosphor can be obtained by employing excess silica over the amount stoichiometrically required, as taught by U.S. Pat. No. 2,254,414 to Roberts, or by incorporating small amounts of magnesium ion in the zinc silicate lattice as disclosed in U.S. Pat. No. 3,416,019 to Kaduk.
Type P-1 phosphors, however, are generally characterized by rapid fluorescence decay and can exhibit the undesirable visual effect of flicker when used in such applications as cathode ray display tubes operated at low refresh rates. To increase the persistence of manganese activated zinc silicate phosphors, small amounts of arsenic are added as taught by U.S. Pat. No. 2,554,999 to Merrill et al. Arsenic-containing manganese activated zinc silicate phosphors are identified commercially as type P-39.
While persistence in P-39 phosphors generally increases with increasing arsenic content, it does so at the expense of brightness. Small increases in arsenic content will often produce appreciable losses in brightness. Thus, in producing P-39 phosphors, the attempt is made to effect a compromise between improved persistence on the one hand, and diminished brightness on the other. Rigorous control of arsenic content of P-39 phosphors during formulation is often difficult, however, owing to the tendency of arsenic compounds to volatilize from the mixture during the repeated high temperature firings required to produce zinc silicate.
Zinc silicate is usually synthesized by a solid-state reaction between ZnO or ZnCO.sub.3 and silicic acid. Typically, as when zinc silicate phosphors are prepared, the reaction between the thoroughly blended dry ingredients is carried out at 1200.degree. C. for aobut 2 hours. Zinc silicate prepared by such methods consists of irregularly shaped, often agglomerated and inter-grown particles ranging from less than 1 .mu.m to several .mu.m in size. The material generally contains a considerable non-crystalline component and the particles show no well-developed faces.
To facilitate the formation of zinc silicate phosphors, it has been the practice to employ alkali metal or alkaline earth metal salts as fluxes as taught by U.S. Pat. No. 2,247,192 to Fonda. The use of such fluxes is not desirable, however, because of the tendency of these metals to form silicates or to quench the beneficial effect of extended fluorescence conferred by the incorporation of arsenic [see for example, Froelich et al. in J. Phys. Chem., 46: 878-885 (1942)].
It is believed therefore, that a green-emitting zinc silicate-based phosphor having enhanced brightness, crystallinity, and uniformity of particle size would be an improvement in the art.